I finally found a source for rivet casings. They can be used for shimming an enlarged pop rivet hole or waterproofing a new pop rivet. See the attached photo if you don't know what they look like. Rick's store doesn't carry them.

Murray's Marine does carry them. Rivet Casing 6-10 (3/16 x 1/2) Part #19-5020 $.70 each.